Glaucoma

Description

Causes

No single factor has been identified as a cause of primary open-angle glaucoma. A number of conditions, alone or in combination, are needed to trigger the processes leading to pressure in the first place and then to the nerve damage that destroys sight. The damage done to the optic nerve in glaucoma is triggered in most cases by the excessive pressure on the optic nerve that, over time, causes damage. Because optic nerve damage occurs in glaucoma patients with normal as well as high intraocular pressure, however, researchers are investigating a number of other abnormal events that occur and can damage the optic nerve.

Genetic Factors

A number of genes have now been identified as possible factors in many cases of glaucoma. A gene called MYOC is of particular interest. Defects in this gene occur in between 3% and 6% of patients with adult-onset and juvenile open-angle glaucoma. They appear to overproduce a sticky protein called myocilin (MYOC), which clogs the trabecular meshwork. (Myocilin is known as a stress protein and is also naturally overproduced during periods of stress.)

Syndromes Associated with Increased Optic Pressure and Glaucoma

Specific syndromes have been identified with glaucoma. Many have an inherited component, although in most cases, other factors must be present to activate the disease process.

Pseudoexfoliation Syndrome. Pseudoexfoliation (PEX) syndrome (also known as exfoliation syndrome) is the most common identifiable condition associated with glaucoma. In one study, 8.5% of patients with open-angle glaucoma had the syndrome. PEX occurs when dandruff-like matter flakes off the outer layer of the lens and collects in the drainage angle. The substance is composed of proteins produced by the lens, iris, and other parts of the eye. People can have this condition and not develop glaucoma, but they are at high risk. In one Australian study, 14.2% of the people with this condition had glaucoma compared to 1.7% of those without exfoliation. PEX has a strong genetic component but other factors (possibly sunlight, an autoimmune response, or slow virus) may be needed to trigger the disease.

Pigment Glaucoma. Pigment glaucoma starts with a condition called pigment dispersion syndrome, an inherited condition in which granules of pigment (the substance that colors the iris) flakes off into the intraocular fluid. In about 30% of cases, these fragments clog the trabecular meshwork and pressure builds up, causing glaucoma. In one study, 1.7% of glaucoma patients had this form of glaucoma.

Irido Corneal Endothelia Syndrome. In irido corneal endothelial syndrome (ICE), cells on the back surface of the cornea spread to the drainage angle, sometimes forming scars that connect the iris to the cornea.

Neovascular Glaucoma. Neovascular glaucoma is always associated with other disorders, usually diabetes, that result in abnormal formation of new blood vessels on the iris and in the drainage system.

Aniridia. Aniridia is a rare inherited disorder (in which the iris is abnormal and increases the risk for glaucoma) that is difficult to treat. (A surgical approach called goniosurgery may help prevent glaucoma in young people with aniridia.)

Congenital Glaucoma. When an infant is born with glaucoma (congenital glaucoma), it is usually caused by an inherited abnormality in the drainage canal. Researchers have identified the gene responsible for 85% of these cases.

Causes of Closed-Angle Glaucoma

People with acute closed-angle glaucoma often have a structural defect that causes a narrow angle between the iris and cornea where the aqueous humor circulates. Conditions that suddenly dilate the pupils may cause this shallow angle to close and precipitate attacks of acute glaucoma in susceptible people. Such conditions may include the following:

• Certain drugs, such as antihistamines, tricyclic antidepressants, certain asthma medications (e.g., nebulized ipratropium), specific anti-seizure agents (e.g., topiramate).
• Darkness.
• Emotional stress.

Causes of Secondary Glaucoma

When intraocular pressure leading to glaucoma is caused by other diseases or conditions, it is known as secondary glaucoma. Secondary glaucoma may be chronic or acute, mild or severe.

Medical Conditions. A number of diseases can contribute to the development of intraocular pressure leading to glaucoma:

• Diseases that affect blood flow to the optic nerve. (Diabetes, high blood pressure, and migraine.)
• Hypothyroidism, which results in a build-up of hyaluronic acid (a gelatinous substance that acts as a lubricant and shock absorber throughout the body) in the trabecular meshwork, which may cause blockage.
• Sleep apnea, a sleep disorder that affects breathing and reduces oxygen.
• Physical injury in the eye.
• Extreme nearsightedness (myopia), which may be due to weakened structures in the eye that increase the risk for glaucoma.
• Previous eye surgery.
• Other disorders, including leukemia, sickle cell anemia, and some forms of arthritis.

Corticosteroids. Corticosteroids, commonly called steroids, have multiple effects on the trabecular meshwork and may even cause genetic changes. In fact, studying the effects of steroids on the eye is helping researchers understand the glaucoma disease process. Steroids pose a higher or lower risk depending on the form:

• Taking topical steroid treatments in the eye poses the highest risk. It must be monitored carefully since, in some cases, damage may be permanent.
• Taking oral corticosteroids, particularly in high doses or for long periods, increases the chance of glaucoma. In such cases, the eye disorder typically develops almost immediately and reverses within two weeks after the drug has been withdrawn.
• Inhaled steroids were not thought to cause glaucoma, but there is some risk in people with a family history of glaucoma and other risk factors.